There are various known technologies for protecting drivers by mounting air bag devices on a motorbike. For example, a technology is known in which an air bag stored in a case mounted on a vehicle body frame is allowed to deploy and inflate by an expanding gas in the event of a frontal collision and, thereby, the driver is restrained (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-137777). This technology allows the possibility of ensuring a large protection area provided by the air bag. However, in the case where an air bag device is mounted on a vehicle body, such as a motorbike, and is configured to open in all directions, the establishment of a technology further effective for reliably restraining the driver with the air bag is highly demanded.